In a steering device of a vehicle, an intermediate shaft is provided between the steering shaft and the steering gear to transmit a rotary operating force applied to the steering wheel to the steering gear side.
Generally, the intermediate shaft adopts the ball spline structure in which plural balls aligned in arrays in the axial direction of the intermediate shaft are interposed in a space between corresponding raceway grooves formed in the inner shaft and in the tubular outer shaft.
Also, an expandable shaft that provides a preload to the rolling elements using an elastic restoring force of the tubular outer shaft has been proposed (for example, see German Unexamined Patent Publication No. 3730393A1, hereinafter “DE '393”).
An expandable shaft that provides a preload to the rolling elements using elastic restoring forces of the outer shaft and the inner shaft by using hollow shafts as both the outer shaft and the inner shaft has been also proposed (for example, see German Utility Model No. 20318654U1).
In the DE '393 reference, a quantity of relative rotation of the inner shaft and the outer shaft is limited by allowing an angular protrusion formed on the outer peripheral surface of the inner shaft to engage with a semicircular concave portion formed on the inner peripheral surface of the outer shaft when high torque is applied.
In the method of allowing the angular protrusion to engage with the semicircular concave portion, however, irregularities of dimensional tolerance among respective components may possibly increase a quantity of relative rotation of the inner shaft and the outer shaft. In such a case, the elasticity of the outer shaft chiefly deteriorates with a long-term use, and the outer shaft may eventually become unable to provide a preload. The absence of a preload gives rise to play between the inner shaft and the outer shaft in the rotational direction, which possibly results in noise.
An object of the invention is therefore to provide an expandable shaft with excellent durability that hardly produces play with a long-term use.